Generally, a color picture tube comprises an envelope constituted by a panel 2 with a substantially rectangular effective portion 1 having a curved inner surface, and a funnel 3 having a funnel shape and joined to the panel 2, as shown in FIG. 3. A phosphor screen 4 having three color phosphor layers which respectively emit blue (B), green (G), and red (R) light beams is formed on the inner surface of the effective portion 1 of the panel 2. In addition, a shadow mask 6 having, on its inner surface, a substantially rectangular and curved effective surface 5 which has a large number of electron beam passage holes for passing electron beams is arranged to oppose the phosphor screen 4.
An electron gun assembly 9 for emitting three electron beams 8B, 8G, and 8R is disposed in a neck 7 of the funnel 3. The three electron beams 8B, 8G, and 8R emitted from the electron gun assembly 9 are deflected by a deflection device 10 mounted on the outer surface of the funnel 3. When the electron beams BB, 8G, and BR pass through the electron beam passage holes of the shadow mask 6 and scan the phosphor screen 4 in the horizontal and vertical directions, a color image is displayed.
Of such color picture tubes, especially, in an in-line color picture tube which emits the three electron beams BB, 8G, and BR arranged in a line on the same horizontal plane, each of the three color phosphor layers of the phosphor screen 4 has a stripe shape elongated in the vertical direction. Accordingly, the shadow mask 6 has electron beam passage hole arrays each having a plurality of electron beam passage holes arrayed in a line along the minor axis direction of the effective surface 5. The plurality of electron beam passage hole arrays are arranged in parallel along the major axis direction of the effective surface 5.
This shadow mask 6 as a color selection electrode originally has a function of landing the three electron beams BB, BG, and BR which have passed through the electron beam passage holes at different angles on the corresponding three color phosphor layers and causing them to emit light. To display an image having a satisfactory color purity on the phosphor screen 4, the three electron beams BB, 8G, and BR which have passed through the electron beam passage holes at different angles must be reliably landed on the corresponding three color phosphor layers.
For this purpose, a predetermined matching relationship must be established between the three color phosphor layers and the electron beam passage holes of the shadow mask 6, and additionally, the matching relationship must be held during the operation of the color picture tube. In other words, the gap between the inner surface of the effective portion 1 of the panel 2, i.e., the phosphor screen 4 and the effective surface 5 of the shadow mask 6, i.e., a so-called q value must always be held within a predetermined allowance.
In the shadow-mask color picture tube, electron beams which pass through the electron beam passage holes of the shadow mask 6 and reach the phosphor screen 4 are 1/3 or less the electron beams emitted from the electron gun assembly 9 because of its operational principle. The remaining electron beams collide with portions other than the electron beam passage holes and are converted into a heat energy to heat the shadow mask 6. As a result, a shadow mask consisting of, e.g., low-carbon steel having a large thermal expansion coefficient expands toward the phosphor screen 4, i.e., causes doming, as indicated by an alternate long and short dashed line in FIG. 4. If doming occurs, the position of an electron beam passage hole 12 changes. When the distance between the phosphor screen 4 and the shadow mask 6 falls outside the allowance, the amount of beam landing shift on a phosphor layer 11 largely changes depending on the luminance and duration of an image pattern to be drawn on the screen. Particularly, when a high-luminance image pattern is locally displayed, local doming occurs, as shown in FIG. 4. The beam landing shifts in a short time, and the landing shift amount increases.
A shadow mask for reducing the landing shift amount is disclosed in Jpn. Pat. Appln. KOKAI Publication No. 08-083573.
For the landing shift due to local doming, an experiment was conducted in which a signal device for generating a rectangular-window-shaped pattern was used to draw a high-luminance pattern 14 having a rectangular window shape on the screen, as shown in FIG. 5, and the beam landing shift amount was measured while changing the shape and position of this high-luminance pattern 14, and the following result was obtained. In this measurement experiment, an elongated high-luminance pattern was drawn with large current beams along the minor axis direction of the screen, i.e., along a vertical axis corresponding to the Y-axis in FIG. 5. This experiment revealed that when the high-luminance pattern was displayed at a position separated, by about 1/3 a width w of the major axis, from the screen center along the major axis, i.e., the horizontal axis corresponding to the X-axis shown in FIG. 5, the beam landing shift was maximized. Especially, the beam landing shift was maximized in an elliptical region 15 at the intermediate portion of the screen shown in FIG. 6. The operational principle has been explained.
In the color picture tube disclosed in Jpn. Pat. Appln. KOKAI Publication No. 08-083573, to minimize the beam landing shift, the interval between electron beam passage hole arrays of the shadow mask 6 is changed depending on the position on the effective surface 5. More specifically, on an orthogonal coordinate system using the center of the effective surface 5 as the origin and the major and minor axes of the effective surface 5 as coordinate axes, an interval PH(N) between an (N-1)th electron beam passage hole array and an Nth electron beam passage hole array from an electron beam passage hole array passing through the central portion of the effective surface 5 toward the periphery of the effective surface 5 along the major axis direction is given by a quartic function of N: EQU PH(N)=A+BN.sup.2 +CN.sup.4
where A, B, and C are quartic functions of a coordinate value y along the minor axis direction respectively, and C temporarily decreases and then increases along with an increase in the absolute value of the coordinate value y.
In this shadow mask 6, the interval between electron beam passage hole arrays which pass through a portion separated from the center of the effective surface 5 by 1/3 the major-axis-direction width w of the effective surface 5 increases near the major axis as the absolute value of the coordinate value in the minor axis direction of the effective surface 5 increases. The interval is set on the basis of the quartic function of the coordinate value y along the minor axis direction on the orthogonal coordinate system, which has an inflection point within the effective surface 5.
However, even when the interval between the electron beam passage hole arrays adjacent to each other is set on the basis of such a quartic function, and the beam landing shift can be reduced, the ratio of the major-axis-direction size of the electron beam passage hole to the interval between the electron beam passage hole arrays is inappropriate because the hole size is defined in accordance with a relatively simple equation. For this reason, when the color picture tube emits light, the image may be dark near a point P3 shown in FIG. 6 and have a color other than white at a point P4, resulting in a degradation in quality of a white image.
In FIG. 7, the interval between the electron beam passage hole arrays on the effective shadow mask surface is defined on the basis of the above-described quartic function. For this reason, the interval is large at a point M2 and small at a point M3. On the other hand, the major-axis-direction size of the electron beam passage hole is defined by a relatively simple quadratic function or the like at the intermediate portion between the screen center and the end of the effective surface such that the hole has an appropriate size at the screen center and at the end of the effective surface. The major-axis-direction size of the electron beam passage hole may be smaller at the point M2 or larger at the point M3 than the appropriate size.
More specifically, at the point M2 where the interval between the electron beam passage hole arrays is relatively large, the major-axis-direction size of the electron beam passage hole becomes small. At the point M3 where the interval between the electron beam passage hole arrays is relatively small, the major-axis-direction size of the electron beam passage hole becomes large. For this reason, the image is dark at the point M2 and bright at the point M3, resulting in luminance irregularity.
Assume that, over the effective surface of the shadow mask 6, the major-axis-direction size of the electron beam passage hole is set in accordance with a simple quadratic or quartic function at four points O, M4, M5, and M6 in FIG. 7. In FIG. 8, the major-axis-direction sizes of electron beam passage holes from the point M1 on the major axis, which is separated from the center of the effective surface of the shadow mask 6 by about 1/3 a major-axis-direction width w' of the effective surface, to the point M2 separated along the minor axis direction by 1/4 a width H of the minor axis is indicated by a grade curve.
When the grade curve of major-axis-direction sizes of electron beam passage holes is represented by a quadratic curve 50 or quartic curve 51, an error from an ideal grade curve 52 is generated at the point M2. When this error is too large or too small with respect to the ideal grade curve 52, the color purity of a white image degrades.